Small world. Your question traversed across the great expanse of
cyberspace, just to end up all the way down here in
Galveston.

As a cellular physiologist, I record electrical brain activity on a nearly
daily basis. However, I use little white lab rats. Thus, I'm not really
an 'expert' on chronic experimentation with humans. But if a general answer
is all your looking for, I think I can help.

Electrode are occassionally chronically implanted in human patients, usually
secondary to some other problem needing surgery such as
epilepsy, or
Parkinson's disease. For these procedures, the electrode is used
for diagnostic purposes, to determine the brain area involved in the
patients' condition. I think it is rare for these electrodes to be implanted
longer than a week or two. Remember, the physician wants to close up
the head before any infection can set in.

Some scientists use monkeys to record brain activity since they are
close cousins to humans, and many brain mechanisms are similar. In these
cases the electrodes can be implanted for much longer periods, up to months
or even years in special cases. The risk of infection is greatest immediately
following the surgical implantation of the electrode, but can always
be a problem. However, application of antibiotic cream is usually sufficient
to treat the infection.